A positive temperature coefficient (PTC) element exhibits a PTC effect that renders the same to be useful as a circuit protection device, such as a resettable fuse. The PTC element includes a PTC polymer material and first and second electrodes attached to two opposite surfaces of the PTC polymer material.
The PTC polymer material includes a polymer matrix that contains a crystalline region and a non-crystalline region, and a particulate conductive filler dispersed in the non-crystalline region of the polymer matrix and formed into a continuous conductive path for electrical conduction between the first and second electrodes. The PTC effect is a phenomena that when the temperature of the polymer matrix is raised to its melting point, crystals in the crystalline region start melting, which results in generation of a new non-crystalline region. As the new non-crystalline region is increased to an extent to merge into the original non-crystalline region, the conductive path of the particulate conductive filler will become discontinuous and the resistance of the PTC polymer material will sharply increase, thereby resulting in electrical disconnection between the first and second electrodes.
Examples of the particulate conductive filler are carbon particles, metal powders, conductive ceramic powders, metalized glass beads, etc. Since carbon particles have lower conductivity, the PTC polymer materials using carbon particles as the particulate conductive filler will have a resistivity greater than 0.1 ohm-cm at room temperature. Hence, for PTC circuit protection devices that require the PTC polymer materials to have a resistivity less than 1.0 ohm-cm or even less than 0.2 ohm-cm, carbon particles are no longer suitable for use alone as the particulate conductive filler.
Although the conductivity of the PTC polymer material can be considerably increased by using the particulate non-carbonaceous particles, such as metal powders, such conductive non-carbonaceous particles having high conductivity tend to result in undesired generation of electric arc within the PTC polymer material during use. The electric arc thus formed could deteriorate the molecular structure of the polymer matrix of the PTC polymer material, which would result in unstable electrical property of the PTC element and reduction in the service life of the PTC element.
U.S. Pat. No. 8,368,504 discloses a PTC polymer material that includes a polymer matrix and a conductive ceramic or metal filler dispersed in the polymer matrix. The polymer matrix is made from a polymer composition that contains at least a primary polymer unit and a reinforcing polyolefin. Although addition of the reinforcing polyolefin in the PTC polymer material can suppress generation of the electric arc in the PTC polymer material, the performance of the PTC polymer material in breakdown and thermal runaway tests is still not satisfactory.